It is realized to extend functionality and to make finer patterning of a variety of sensors by combining a piezoelectric element to a micro electro mechanical system (MEMS).
JP 2008-206333A describes a piezoelectric thin film device equipped with a substrate, a piezoelectric thin film having a diaphragm structure supported by the substrate, and a thin film electrode which deforms the surface shape of the piezoelectric thin film by impressing voltage to the piezoelectric thin film. A multilayer film is layered on the piezoelectric thin film to make the surface flat. Furthermore, a reflective film layer which reflects light is formed on the multilayer film.
The piezoelectric thin film device is used, for example, in a MEMS scanner which projects a picture on a screen by reflecting laser light by a reflective film layer. The reflective film layer in the piezoelectric thin film device functions as a mirror part which reflects laser light. Since the reflective film layer is deformed by a deformation of the piezoelectric thin film, the focal length of a mirror part can be controlled. For this reason, it becomes possible to draw a picture with depth.
The reflective film layer is required to be flat to raise the image quality of the picture projected on the screen. The flatness of the reflective film layer is dependent on a flatness of an interface with a layer directly under the reflective film layer. The multilayer film of JP 2008-206333A is formed between the piezoelectric thin film and the reflective film layer. However, the multilayer film is formed by photoresist, and the crystallinity of the interface with the reflective film layer is not taken into consideration. For this reason, the flatness is insufficient as a piezoelectric thin film. In other words, the arithmetic average coarseness of the surface of the piezoelectric thin film cannot be made small enough.
Daisuke Akai et al, Journal of Crystal Growth 259 (2003), 90-94 describes a lead zirconate titanate (PZT) film as a piezoelectric thin film which has crystal orientation. The PZT film is formed by sol gel process. Specifically, the PZT film is formed to grow on the surface of Al2O3 (001)/Si (001) having an orientation of (001).
However, an epitaxial film which has sufficient crystallinity cannot be formed by the sol gel process. Specifically, a domain area which shows a predetermined orientation on the surface of the piezoelectric thin film cannot be enlarged enough. That is, the arithmetic average coarseness is raised by much grain boundary.